Continuous casting installation comprising a soft reduction section

ABSTRACT

The invention relates to a method and a device for the continuous casting of slabs or ingots, in particular of thin slabs in a continuous casting installation. Said installation comprises a soft reduction section in a continuous casting guide under the mould. The soft reduction section contains pressure rollers and support roller ( 3, 4 ), which are continuously restrained in relation to one another, either individually or as a segment ( 1 ), by means of hydraulic cylinders ( 7, 7′ ) and are restrained in a limiting manner by stops ( 30 ). The installation uses a hard-pressure restraining force in an area of the soft reduction section that has not yet completely solidified and a soft-pressure restraining force in an area of the soft reduction section that has completely solidified. Threshold and changeover values for the hard and soft-pressure are defined in the segment is restrained using hard-pressure, such a way that if the restraining force lies below the threshold or changeover value, and if the restraining force lies above the threshold or changeover value the segment is restrained using soft-pressure.

The invention relates to a method of an apparatus for continuous castingof slabs and ingots, in particular, of thin slabs in a continuouscasting installation including a soft reduction section in a strandguide under a mold and having pressure rollers and support rollers whichare continuously restrained relative to one another, either individuallyor as a segment by hydraulic cylinders, and are restrained by stops in alimiting manner.

Many presently operating continuous casting installations and continuouscasting installations the operation of which is to begin, in particular,continuous casting installations for casting thin slabs use anoperational technology with soft reduction. The method and acorresponding apparatus are primarily installed in the horizontalportion of a strand guide and serve for improving the microstructuralquality and the surface quality of the cast products.

A soft reduction effect is achieved by a reduction of a strand thicknessin the region of its solidification with a molten core up to the tip ofa liquid crater. The region, in which the soft reduction significantlyaffects the strand inner quality lies in the solidification range at FS:0.3 through 0.8 according to a longitudinal position on thesolidification path. The soft reduction, which consists in reduction ofthe thickness in small steps in the above-mentioned region, prevents toa substantial extent segregation in the melt.

In order to achieve the soft reduction in a defined operational regionat different operation conditions, which can be caused, e.g., by changesin the casting speed, or by stoppages, the zone, in which the strandthickness is reduced, should be changeable along the longitudinalposition of the strand. To this end, a position controlled powercylinder is used in the support segments.

In order to effect a predetermined reduction of the thickness of arunning strand, the support rollers are conically adjusted to a positionfavorable for soft reduction by determining the position of the craterand by determining, by calculations or by measurements, the change inthe pressure applied to the pressure rollers by the strand. This methodis practically completely developed and is designated as a dynamic softreduction that, however, because of the used technical means, isconnected with high costs.

An object of the invention, proceeding from the above-mentioned state ofthe art, is a method and an apparatus for casting slabs or ingots, inparticular, thin slabs, which is recited in the preamble of claim 1 andwhich would make possible a highly effective soft reduction with reducedcosts of the technical means and with a high quality of the results.

In order to achieve the above-mentioned object, according to theinvention, hard-pressure restraining force is used in an area of thesoft reduction section that has not yet completely solidified, and asoft-pressure restraining force is used in an area of the soft reductionsection that has completely solidified, with a threshold or changeovervalue for the hard-pressure and the soft-pressure being defined and witha restraining force being below the threshold or changeover value,restraining of the segment is effected by using the hard-pressure, andabove the threshold or changeover value, restraining of the segment iseffected by using the soft pressure.

The inventive process is based on the knowledge obtained from apractical operation that a completely solidified section of a strand, incase it reaches the region of the soft reduction, should not be deformedas, otherwise, the support or pressure rollers, or their bearings, orthe segment frame would be damaged, in particular when the sortreduction region is formed of several conically adjustable segments.Such a case can exist when the casting speed is changed and/or when theoperation is interrupted. In such a case, the restraining force for thesoft reduction must be able to change from the hard-pressure to thesoft-pressure, amounting advantageously to 40% of the hard pressure inrun-out section of the strand.

According to the invention, this changeover from the hard-pressure tothe soft-pressure and, vice versa, if needed, takes place automaticallyas a result of sensing of a pressure increase pulse generated in thepower cylinder upon entrance of a non-deformable, completely solidifiedstrand in the conically adjusted segment and which is sensed by apressure sensor.

According to one embodiment of the method, it is contemplated that at arunning cast operation with the hard-pressure in the soft reductionsection, upon reaching an upper threshold or changeover value of therestraining force, a changeover to the soft-pressure take place and,vice versa, at a running cast operation with the soft pressure in thesoft reduction section, upon reaching a lower threshold or changeovervalue for the restraining force, a changeover to the hard-pressure takesplace.

According to a further development of the inventive method, upon entryof a non-deformable, completely solidified strand section in a conicallyadjusted, for a hard pressure, segment, a changeover from a hardpressure to a soft pressure takes place, based on a resulting pressureincrease pulse in the power cylinder.

The power cylinders are primarily subjected to a soft reductionpressure, a so-called hard-pressure, with each displacement of thepiston resulting in a pressure change. A pressure sensor on the cylinderprovides for the use of the pressure increase pulse, which is generatedin the cylinder, as discussed above, when a non-deformable, completelysolidified strand enters the conically adjusted seament for automaticswitching to a soft-pressure. For practically effecting this, a shut-offvalve is partially open by a relief valve so that a segment in thehydraulic switching circuit can give way, i.e., the segment inlet widthchanges so that the completely solidified strand section can passthrough the segment at the soft-pressure.

According to a further embodiment of the inventive method, uponswitching of a segment to an operation condition with a soft-pressurefor a completely solidified strand section in the segment, a pressureinquiry is initiated wherein in short, cyclic, time intervals,changeover from the soft pressure to the hard pressure and vice versatakes place, and it is checked whether the pressures remain unchanged orwhether they are above or below a respective threshold or changeovervalue and, dependent on a pressure test an a adjustment to the hardpressure or the soft-pressure takes place. These measures prevent afurther opening of the segment by a ferrostatic pressure of the melt.

According to a still further embodiment of the inventive method, allsegments in the soft reduction region and the segments behind them up tomachine end are subjected to a cyclic pressure inquiry. Thereby, auniform quality of a cast product is insured.

According to a preferred embodiment of the method, in particular in ahorizontal portion of a continuous casting installation, a strandsection, in which soft reduction conditions are true for an operationstate of the casting operation and for used steel types, is determined.Then, in the determined strand support section, all support segments areindividually adjusted with a necessary conicity in such a way that arunning-through strand retains a necessary for the soft reduction,deformation.

To this end, the hydraulically adjustable support segments areindividually adjusted, with respect to a thickness predetermined by amechanical stop, to a given amount of soft reduction. The mechanicalstops prevent the support rollers from approaching to close to eachother and thereby any damage of the support structure.

An apparatus for continuous casting of slabs and ingots, in particular,of thin slabs in a continuous casting installation including a softreduction section in a strand guide under a mold and having pressurerollers and support roller which are continuously restrained relative toone another, either individually or as a segment by hydraulic cylinders,and are restrained by stops in a limiting manner, includes a segmentwith support and pressure rollers adapted to a strand, driven with anadjustable speed, and arranged in a segment frame with frame upper partand frame lower part, and a hydraulic cylinder. According to theinvention, a hydraulic circuit includes a pressure sensor and a pressurechangeover valve operated by it, a shut-off valve arranged in itsswitching circuit, a control valve and a relief valve, means forpreventing rupture of conduits and including a pressure relief valvewith a check valve. The apparatus serves to provide a hard-pressurerestraining force within the not yet completely solidified region of thesoft reduction section or a soft-pressure restraining force within thecompletely solidified region of the soft reduction section and forswitching from the hard pressure to the soft-pressure and vice versa.The restraining force is uniformly led into the pressure and supportrollers from the hydraulic cylinder through the frame upper part and theframe lower part.

Further particularities, features, and advantages of the invention willbecome apparent from the following description of an embodiment of theinvention shown schematically in the drawing. The drawings show:

FIG. 1 a section of a support segment for a cast strand, together withan associated circuit diagram for effecting the method according to theinvention;

FIG. 2 a support segment in an open position for running in of a strandwith a partially liquid core in the region of the tip of the crater.

FIG. 1 shows a section of a support segment 1 for a strand 2, e.g., of athin slab cast in a continuous casting installation. The support segmentis located within a soft reduction section of a strand guide and haspressure rollers 3 and support rollers 4 which are continuouslyrestrained relative to one another, either individually or, in the caseconsidered here, as a segment, with a hydraulic cylinder 7.

The numeral 30 designates a mechanical stop which defines apredetermined thickness and against which a segment lower part 6 can beindividually adjusted relative a segment upper part 5 to an amount ofsoft reduction by pulling the piston rod of the cylinder 7.

In the soft reduction section, the thickness of the cast thin slab isreduced in small steps in order to prevent segregation within the strandand in order to improve the surface quality. The soft reduction sectionincludes at least large elongate regions of the strand with molten coretip to the tip of the liquid crater. In order to adapt the adjustment ofthe support segments to a respective changing position of the cratertip, which is changed as a result of different operational conditions, aspecial hydraulic circuit is provided.

As shown in the circuit diagram of FIG. 1, the working pressure of thehydraulic circuit is tapped above and beneath the piston of thehydraulic cylinder 7 and is evaluated by a pressure sensor 10. Thepressure sensor 10 on the cylinder 7 provides for use of the pressureincrease pulse in the cylinder, e.g., for switching from hard-pressureto soft-pressure. To this end, the pressure sensor 10 is connected witha pressure changeover valve 11 by signal lines.

The power cylinder 7 is subjected to the action of, primarily, the softreduction pressure, i.e., to the action of hard-pressure, and isconnected, by a shut-off valve 12, e.g., conduit rupture-preventingvalve or a pilot-controlled check valve, with a pressure relief valve 15in such a way that each displacement of the piston results in a pressurechange which, as it has already been mentioned above, in case of apressure increase in the cylinder, is sensed by the pressure sensor andis used for switching to a soft pressure.

In case of soft pressure, on the other hand, the shut-off valve 12 isopen by a relief valve 14, and the segment can give in, i.e., thesegment inlet width can be changed, whereby the solidified strand 2 canpass through the segment 1 under soft pressure.

When at another point in time, a non-solidified strand region with afluid crater reaches the segment 1, the segment 1 is subjected to anaction of soft pressure and gives further in under the action of theferrostatic pressure of the strand 2, opening even more, which is notacceptable and which causes a reduction in quality of the cast strand.

In order to prevent a further opening of the segment 1 under theferrostatic pressure of the non-solidified strand 2, a pressure inquiryin initiated in the hydraulic circuit, at which in short time intervalswith time pulses, the control valve 13 is switched, by the pressuresensor 10, from soft pressure to hard pressure, and the shut-off valve12 is closed.

Finally, it is checked whether the hard-pressure remains constant, i.e.,whether it remains under the changeover valve or exceeds it. If thepressure is below the changeover value, it means that the strand 2 hasnot yet completely solidified and a hard pressure is needed forrestraining the segment.

When, however, the pressure exceeds the changeover value, it means thatthe strand 2 has completely solidified, and the segment restrain shouldbe effected with soft pressure.

FIG. 2 shows a segment 1 in a position for running of the strand 2, withstrand shells 2′ not yet closed with a core 8 of liquid melt, in. Thesegment frames include the segment frame-upper part 5 and the segmentframe-lower part 6 which are clamped together with a relatively largeforce against the action of the ferrostatic pressure of the strandshells 2′ and the liquid molter core 8. The segment 1 is equipped withpressure guide rollers 3 and 4 which are partially provided with drives,as is known.

The hydraulic cylinder 7′ is subjected to the action of the softpressure, and the hydraulic cylinder 7 is subjected to the action of thehard-pressure, in accordance with the present invention.

1. A method of continuous casting of slabs and ingots, in particular, ofthin slabs in a continuous casting installation including a softreduction section in a strand guide under a mold and having pressurerollers and support rollers (3,4) which are continuously restrainedrelative to one another, either individually or as a segment (1) byhydraulic cylinders (7, 7′), and are restrained by stops (30) in alimiting manner, the method comprising the steps of using ahard-pressure restraining force in an area of the soft reduction sectionof the strand guide in which the strand has not completely solidified,and using a soft-pressure restraining force in an area of the softreduction section of the strand guide in which the strand has completelysolidified; defining a pressure changeover value at which restrainingwith the hard-pressure restraining force is switched to restraining withthe soft-pressure retaining force and vice versa; and applying the softpressure restraining force when the pressure is above the changeovervalue, and applying the hard-pressure retaining force when the pressureis below the changeover value.
 2. A method according to claim 1,comprising the steps of changing the hard-pressure restraining force tothe soft-pressure restraining force when an upper threshold of therestraining force is reached, and changing the soft-pressure restrainingforce to the hard-pressure restraining force when a lower threshold ofthe restraining force is reached.
 3. A method according to claim 1,comprising the step of changing the hard-pressure restraining force tothe low-pressure restraining force upon entry of a non-deformable,completely solidified strand section in a conically adjusted, for a hardpressure, segment, in response to a resulting pressure increase pulse inthe power cylinder.
 4. A method according to claim 1, comprising thesteps of initiating, after switching of a segment to an operationalconditional with a soft-pressure for a completely solidified strandsection, in case of a following feeding of a non-completely solidifiedstrand section in the segment, a pressure inquiry wherein in short,cyclic, time intervals, a changeover from the soft-pressure to thehard-pressure and vice versa takes place; checking whether the pressuresremain unchanged or whether they are above or below a respectivechangeover value; and, dependent on a pressure test, effectingadjustment to the hard pressure or the soft pressure.
 5. A methodaccording to claim 4, comprising the step of subjecting to the pressureinquiry all of segments in the soft reduction regions and of all of thesegments up to a casting machine end.
 6. A method according to claim 1,comprising the step of determining, in a horizontal portion of thecontinuous casting installation, a strand section in which softreduction conditions are true for an operational state of the castingoperation and for used steel types.
 7. A method according to claim 6,comprising the step of individually adjusting, with a necessaryconicity, all support segments in a determined strand section support insuch a way that a running-through strand retains a deformation necessaryfor soft reduction.
 8. A method according to claim 1, comprising thestep of individually adjusting the hydraulically adjustable supportsegments with respect to a thickness predetermined by a mechanical stop(30), to a redetermined amount of soft reduction.
 9. A method accordingto claim 1, comprising the step of adjusting the soft pressure aboutbetween 30% and 50% of the hard pressure.
 10. A method according toclaim 9, wherein the soft pressure adjusting step includes adjusting thesoft pressure to 40% of the hard pressure.
 11. An apparatus forcontinuous casting of slabs and ingots in a continuous castinginstallation including a soft reduction section in a strand guide undera mold and having pressure rollers and support rollers (3,4) which arecontinuously restrained relative to one another, either individually oras a segment (1) by hydraulic cylinders (7,7′), with a change of therestraining pressure and are restrained by stops (30) in a limitingmanner, the apparatus comprising: a hydraulic circuit including apressure sensor (10) for determining pressure in the hydraulic circuit;a pressure changeover valve (11) controlled by the pressure sensor (10);a shut-off valve (12); a control valve (13) for controlling flow to thehydraulic cylinders (7, 7′); a relief valve (14) for controlling theshut-off valve (12); and a pressure relief valve (15) cooperating withthe shut-off valve (12) for providing a hard pressure restraining forcewithin one of a region of the soft reduction section in which the strandhas not yet completely solidified, and a region of the soft reductionsection in which the strand has completely solidified, and for switchingfrom the hard pressure to the soft pressure and vice versa.